Composite sensor

ABSTRACT

A combined gas sensor of the invention is equipped with a second oxygen sensing electrode 5 active only to oxygen, which is disposed on a solid electrolyte substrate 3a such that it is directly exposed to detection gases. Sensor disorder is detected by measuring an electromotive force (signal output 2) generated between a reference electrode 7 and the second oxygen sensing electrode 5 as an oxygen concentration in an exhaust gas atmosphere, and comparing and judging the detected result with a signal output 1 corresponding to an electromotive force between a first oxygen sensing electrode 4 and the sit reference electrode 7, or with a signal output of an oxygen pump current.

TECHNICAL FIELD

The present invention relates to a gas sensor, and particularly to acombined gas sensor for detecting the concentration of acid gases suchas nitrogen oxides, etc. in exhaust gases and oxygen concentration in agas atmosphere to be detected.

BACKGROUND OF THE INVENTION

Various poison gases including nitrogen oxides (NO_(x)), hydrocarbongases (HC), sulfur oxides (SO_(x)), CO, CO₂, and etc., are exhaustedfrom internal combustion engines including those of automobiles, andcombustion instruments of thermal power plants, and factory plants, etc.They are called environmental gases, and severe legal regulations havebeen enforced to limit the emission of environmental gases. Thus, thedevelopment of a gas sensor capable of detecting the low concentrationsof the exhausted environmental gases at a low cost has been demanded.

Recently, an all-solid-type gas sensor which can carry out thecontinuous measurement by direct insertion in exhaust gases from enginesof automobiles, etc., has been watched with keen interest, and manyinvestigations and developments thereof have been reported. The presentinventors already proposed a mixed potential-type NO_(x) sensor capableof measuring the total NO_(x) concentrations in exhaust gases at realtime. For example, in Japanese Patent Laid-Open Publication No.201942/1999, a mixed potential-type NO_(x) sensing electrode is disposedin a gas chamber formed by Zirconia solid electrolyte, and also a NO_(x)converting electrode is disposed in the same chamber such that it facesto the NO_(x) sensing electrode. That is, in this structure, NO_(x) (NOand NO₂) in exhaust gases are electrochemically converted to a NO₂single substance gas, and the converted NO₂ concentration is detected bythe NO_(x) sensing electrode as the total NO_(x) concentration. In theembodiment of Japanese Patent Laid-Open Publication No. 201942/1999, anoxygen pump (converting pump) for converting NO_(x) in the gas chamberto NO₂ and another oxygen pump for controlling the oxygen concentrationin a gas chamber are disposed. Also, a reference electrode to the NO_(x)sensing electrode described above and an oxygen sensing electrode formeasuring the oxygen concentration in the gas chamber are disposed inthe same gas chamber. By referring the potential of the oxygen sensingelectrode due to the oxygen concentration in the gas chamber as areference potential of the NO_(x) sensing electrode, the output of theNO_(x) sensor is hardly influenced even when the oxygen concentration inthe gas chamber is fluctuated.

The fluctuation of the oxygen concentration is, however, large and sharpin the exhaust gases from e.g. an automobile. Therefore, in the NO_(x)sensor element with the gas chamber structure shown in Japanese PatentLaid-Open Publication No. 201942/1999, etc., it is necessary to make theoxygen concentration in the gas chamber at a level at which interferencegases such as HC, CO, etc., entering the gas chamber together withNO_(x), are oxidized to non-interference gases. The oxygen concentrationaround the sensing electrode based on mixed potential should also besustained at a substantially constant value of several percents.Accordingly, the occurrence of disorder of a system, in which thedetection of the oxygen concentration in a gas chamber is carried outand the oxygen pump driving is controlled by feeding back the signal ofthe oxygen concentration, gives a large error to the accuracy of thesensor output. Also, as a precondition, it is necessary that the gastightness in the gas chamber and the air duct is kept in order.

On the other hand, the temperature of the exhaust gases from automobilesis increasing more and more recently, and the gas temperature changeslargely, depending on the driving states of engine thereof. Also, poisoncomponents for the electrodes are largely contained in exhaust gases,and further, the vibration of the body or the engine of automobiles islarge. Thus, an improvement of the reliability of the gas sensor used inexhaust gases under such severe circumstances becomes very important.

However, in the conventional NO_(x) sensor disclosed in Japanese PatentLaid-Open Publication No. 201942/1999, even when disorder occurs in thesystem in which the detection of the oxygen concentration in the gaschamber is carried out, and the oxygen pump driving is carried out byfeed-back control, it was impossible to self-diagnosed diagnose whetherthe sensor was in order or not. Also, when the gas tightness of the gaschamber and the air duct, is lost during the use to influence the sensoroutput, the NO_(x) sensor did not have any means for informing a userwhether or not such is due to the sensor disorder. That is, theabove-described NO_(x) sensor has problems that when deterioration ofelectrodes, disorder in controlling the oxygen concentration in the gaschamber, and losing of the gas tightness of the gas chamber or the airduct occur during the use of the gas sensor, it cannot be determinedthat they are due to disorder of the sensor or not.

BRIEF SUMMARY OF THE INVENTION

The present invention has been made for solving the above-describedproblems.

According to the invention, there is provided a combined gas sensorhaving a structure equipped with a first gas chamber formed by an oxygenion conductive solid electrolyte; a first gas inlet having a diffusionresistance for restricting the flow amount of gases entering the firstgas chamber; a first oxygen sensing electrode active only to oxygen,which is disposed on a surface of the first electrolyte substrate insidethe first gas chamber; a gas sensing electrode active to at least targetgases and oxygen, which is disposed on the same surface of the firstelectrolyte as the first oxygen sensing electrode; a reference electrodeactive to oxygen, which is disposed on the first solid electrolytesubstrate such that it is brought into contact with an atmospheric airseparated from the first gas chamber; an oxygen pump cell forcontrolling oxygen concentration in the first gas chamber, of whichelectrodes are made of materials active only to oxygen and disposed on asecond solid electrolyte substrate facing to the first solid electrolytesubstrate; a gas reforming pump cell for chemically or electrochemicallyconverting or decomposing the gases, of which a gas reforming electrodeis active to oxygen and the target gases, and disposed on the secondsolid electrolyte substrate in the first gas chamber so as to face tothe gas sensing electrode, and a gas reforming counter electrode isdisposed on the second solid electrolyte substrate outside the first gaschamber such that it is brought into contact with the atmospheric air;and a second oxygen sensing electrode active only to oxygen, which isdisposed on the first solid electrolyte substrate such that it isdirectly exposed to the detection gas atmosphere.

Furthermore, according to the invention, there is provided a combinedgas sensor comprising an oxygen pump cell for controlling the oxygenconcentration in the first gas chamber, of which the electrodes are madeof materials active only to oxygen and disposed on the second solidelectrolyte substrate facing to the first solid electrolyte substrate;and a gas reforming pump cell for chemically or electrochemicallyconverting or decomposing the gases, of which a gas reforming electrodeis active to oxygen and the target gases and disposed on the secondsolid electrolyte substrate in the first gas chamber so as to face tothe gas sensing electrode, and a gas reforming counter electrode isdisposed on the second solid electrolyte substrate outside the first gaschamber such that it is brought into contact with the atmospheric air,and characterized by having driving cell circuits wherein a DC variablepower supply for controlling the oxygen concentration in the first gaschamber at a constant level by feed-back control of pumping currents forthe oxygen pump cell, according to an electromotive force between theoxygen sensing electrode and the reference electrode as a signal outputof the oxygen concentration in the first gas chamber, and a DC constantvoltage power supply for making oxygen pumping currents of the gasreforming pump cell constant, and thus making an electrochemicaldecomposition or conversion amount of the target gas constant; areconnected to the oxygen pump cell and the gas reforming pump cell,respectively.

According to the invention, in the gas sensor having the gas chamberformed by the oxygen ion conductive solid electrolyte, the oxygenconcentration in the gas chamber or the oxygen pumping current isreferred to the oxygen concentration in the detection gas atmosphere inorder for comparison and judgment, whereby the disorder of the gassensor element can be self-diagnosed. Thus, the operation state of thegas sensor can be always automatically monitored. In particular, whenthe invention is applied to a gas sensor for mounting on an automobilewith severe environment, the reliability of the sensor is greatlyimproved. Furthermore, because according to the gas sensor of theinvention, the oxygen concentration in the detection gas atmosphere canbe directly measured, when the gas sensor is, for example, a NO_(x)sensor for mounting on an automobile, the concentration of NO_(x) andoxygen in the exhaust gases can be simultaneously measured.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional structural view showing an embodiment of thesensor of the invention;

FIG. 2 is a cross-sectional structural view showing another embodimentof the sensor of the invention;

FIG. 3 is a cross-sectional structural view showing a still anotherembodiment of the sensor of the invention;

FIG. 4 is a circuit principle view showing an embodiment of theself-diagnosis method of the sensor of the invention;

FIG. 5 is a diagnosis logic showing an embodiment of the self-diagnosismethod of the sensor of the invention;

FIG. 6 is a diagnosis logic showing another embodiment of theself-diagnosis method of the sensor of the invention;

FIG. 7 is showing the characteristic curves used for the self-diagnosismethod of the sensor of the invention;

FIG. 8 is a diagnosis logic showing a still another embodiment of theself-diagnosis method of the sensor of the invention;

FIG. 9 is showing another characteristic curves used for theself-diagnosis method of the sensor of the invention; and

FIG. 10 is a cross-sectional structural view showing an even anotherembodiment of the sensor of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A fundamental structure of the gas sensor of the invention is shown inFIG. 1. The sensor structure comprising; a first gas chamber 1 formed bytwo oxygen ion conductive solid electrolyte substrates 3 a, 3 b; a firstgas inlet 11 having a gas diffusion resistance for restraining the flowamount of gases entering the first gas chamber; a first oxygen sensingelectrode 4 active only to oxygen, which is disposed on a surface of thefirst oxygen ion conductive solid electrolyte substrate 3 a inside thefirst gas chamber; a gas sensing electrode 6 active to at least targetgases and oxygen, which is disposed on the same surface of the firstelectrolyte substrate as for the first oxygen sensing electrode insidethe first gas chamber; a reference electrode 7 active to oxygen, whichis disposed on the first solid electrolyte substrate 3 a such that it isbrought into contact with an atmospheric air separated from the firstgas chamber 1; an oxygen pump cell (8 a, 8 b) for controlling oxygenconcentration in the first gas chamber, of which electrodes are made ofmaterials active only to oxygen and disposed on the second solidelectrolyte substrate 3 b facing to the first solid electrolytesubstrate; a gas reforming pump cell (9 a, 9 b) for chemically orelectrochemically converting or decomposing the gases, of which a gasreforming electrode 9 a is active to oxygen and the target gases anddisposed on the second solid electrode substrate 3 b in the first gaschamber so as to face to the gas sensing electrode 6, and a gasreforming counter electrode 9 b is disposed on the second solidelectrode substrate 3 b outside the gas chamber such that it is broughtinto contact with the atmospheric air; and a second oxygen sensingelectrode 5 active only to oxygen, which is disposed on the first solidelectrolyte substrate such that it is directly exposed to the detectiongas atmosphere.

The gas sensor of this structure is described in detail in the case of acombined NO_(x) sensor which is a specific embodiment for use. Theabove-described NO_(x) sensor shown in FIG. 1 can detect total NO_(x)concentration in the exhaust gases of automobile.

First, reducing gases such as HC (hydrocarbon gases) and CO among NO_(x)(NO and NO₂), HC, CO, CO₂, SO_(x), H₂O, O₂, etc., which are contained inthe exhaust gases diffusing and flowing in the first gas chamber 1through the gas inlet 11, are oxidized to non-interference gases bypumping in and out oxygen through an air duct 13 using the oxygen pumpcell and keeping the atmosphere inside the first gas chamber 1 at apredetermined oxygen concentration. Because the gas sensor is usuallyheated and operated at 500° C. or higher, the reducing gases such as HCand CO are instantly oxidized to non-interference gases such as CO₂ andH₂O due to catalytic effects of the solid electrolyte constituting thegas chamber and the electrode materials.

Consequently, NO in the NO_(x) is electrochemically converted into NO₂by a NO_(x) converting electrode which the gas reforming electrode 9 a.The NO_(x) converting electrode 9 a is active to NO_(x) and oxygen, andNO can be converted into NO₂ at least on the electrode. By using theNO_(x) converting electrode 9 a disposed in the first gas chamber 1 as acathode, and the NO_(x) converting counter electrode 9 b disposed in theair duct 13 leading to the atmospheric air as an anode, and by applyinga predetermined potential thereto, the gas sensor is operated. TheNO_(x) converting cell with the NO_(x) converting electrode 9 a and itscounter electrode 9 b is fundamentally the same as the oxygen pump cell(8 a, 8 b), but is different in the activity to NO_(x) of the electrodedisposed in the first gas chamber.

In succession, NO_(x) converted into NO₂ by the NO_(x) convertingelectrode is detected as a mixed potential by the NO_(x) sensingelectrode 6 facing to the NO_(x) converting electrode. The output ofNO_(x) in this case is obtained by measuring a potential differencebetween the reference electrode 7 and the NO_(x) sensing electrode 6 ora potential difference between the No_(x) sensing electrode 6 and thefirst oxygen sensing electrode 4. That is, the total NO_(x)concentrations in the exhaust gases can be detected by the sensor of theinvention without any influence of the interference gases such as HC,etc.

In the sensor structure shown in FIG. 2, a gas reforming catalyst layer10 is disposed in place of the gas reforming pump cell. NO_(x) in theexhaust gases can be converted to a NO gas as an equilibrium state forthe detection of the total NO_(x) by increasing the temperature of thesensor and setting the oxygen concentration at a low level. That is, thesensor has such an effect that NO_(x) in the exhaust gases is convertedinto a NO gas in the first gas chamber 1. The converted NO is detectedas a mixed potential by the above-described NO_(x) sensing electrode 6.With regard to detecting the disorder of the sensor, in this case, it isthe same as that of NO₂ sensing case.

As described above, in the sensor structure shown in FIG. 1 or FIG. 2,it is necessary to maintain the oxygen concentration in the first gaschamber 1 at a predetermined (oxygen) concentration of a several % levelfor oxidizing and removing the interference gases such as HC, etc. andobtain a reliable mixed potential output. For this purpose, anelectromotive force due to the oxygen concentration difference betweenthe oxygen sensing electrode 4 disposed in the first gas chamber 1 andthe reference electrode 7 is detected, and the signal output 1 (thevoltage output at V₀₂ in the figures) is used for feedback-control ofthe driving voltage of the oxygen pump cell (8 a, 8 b). For example,when the oxygen concentration in the first gas chamber 1 is set up at2%, the setting voltage is −43 mV at 600° C.

When the signal output 1 detecting the oxygen concentration in the firstgas chamber 1 becomes an reliable state due to some disorder of thesensor, a large error occurs in the NO_(x) concentration output of theNO_(x) sensor. That is, in the case that the oxygen concentration in thefirst gas chamber 1 is abnormally lowered, the NO_(x) sensing output islargely lowered because HC in the exhaust gases reaches the NO_(x)sensing electrode without being sufficiently oxidized and removed. TheNO₂ sensitivity of the NO_(x) sensing electrode is also changed inproportion to the logarithm of the oxygen concentration. Accordingly,the disordered lowering of the oxygen concentration in the gas chamber 1increases the NO_(x) sensing output. On the other hand, in the case thatthe oxygen concentration in the first gas chamber 1 abnormallyincreases, the NO_(x) sensing output is lowered owing to the dependenceon oxygen concentrations. One of the causes of the disorder in thesensor described above is, for example, deterioration of the oxygensensing electrodes 4, 5 by poisoning and so on. In this case, the oxygenpumping out or in is liable to occur excessively even in a predeterminedoxygen concentration because the detection potential (signal output 1, 2), is changed. In the case that the gas tightness of the air duct 13 islost, the oxygen pumping capability is lowered and the oxygenconcentration in the first gas chamber does not reach a predeterminedvalue. Furthermore, in the case that the gas tightness of an air duct 14is lost, the potential of the reference electrode 7 is usually lowereddepending on the oxygen concentration in the exhaust gases. Thus, anabsolute value of the electromotive force between the referenceelectrode 7 and the oxygen sensing electrode 4 in the first gas chamber1 is lowered resulting in disorder of controlling oxygen pumping.

Therefore, in the sensor structure of this invention shown in FIG. 1 orFIG. 2, the second oxygen sensing electrode 5 active only to oxygen isdisposed on the first solid electrolyte substrate 3 a, which is directlyexposed to the detection gas atmosphere. The electromotive force (signaloutput 2) between the reference electrode 7 and the second oxygensensing electrode 5 is detected as the oxygen concentration in theexhaust gas and compared with the above-described signal output 1 or thesignal output of the pumping current of the oxygen pump cell, wherebythe disorder of the sensor can be detected.

In regard to a specific diagnosis method of the gas sensor carrying outthe oxygen concentration control in the gas chamber 1 shown in FIG. 1 orFIG. 2, the circuit principle of diagnosis for detecting the disorderillustrated in FIG. 4 is proposed. The diagnosis circuit principle shownin FIG. 4 is constructed by four portions: the first portion is an inputand voltage comparing portion; the second portion is a controllingportion for the diagnosis mode; the third portion is a signal operatingportion; and the fourth portion is an output portion of diagnosisresults. A current signal

V_(ip) of the oxygen pumping cell (8 a, 8 b) is defined as signal I; anoxygen concentration signal V₀₂ in the first gas chamber 1 as signal II;and an oxygen concentration signal V*₀₂ in the detection gas atmosphereas signal III, respectively. When the oxygen concentration signal V₀₂ inthe first gas chamber 1 is equal to a predetermined setting value V_(s),i.e., the output from a window-comparator is zero (V₀₂=V_(s)) thediagnosis mode is defined as diagnosis-A.

On the other hand, when the oxygen concentration in the first gaschamber 1 is different from the predetermined setting value, i.e., theoutput from the window comparator is not zero (V₀₂≠V_(s)), the diagnosismode is defined to be diagnosis-B. In the case of the diagnosis- A, whenthe pumping current of the oxygen pump is zero, and the oxygenconcentration in the first gas chamber 1 is equal to that in thedetection gases (V₀₂=V*₀₂), the operation of the sensor system is judgedto be in order. Notwithstanding the oxygen pumping current is zero, onthe contrary, the operation of the sensor system is judged to be indisorder and the result is given when the oxygen concentration in thefirst gas chamber 1 is different from that in the detection gases(V₀₂≠V*₀₂). Similarly, notwithstanding the oxygen concentration V₀₂ inthe first gas chamber 1 is equal to the oxygen concentration in thedetection gases (V₀₂=V*₀₂), the operation of the sensor system is judgedto be in disorder and the result is given when the oxygen pumpingcurrent is not zero.

Next, the cases of the diagnosis-B are considered. When the oxygenconcentration signal V₀₂ in the first gas chamber 1 is lower than apredetermined oxygen concentration setting value V_(s), the oxygenpumping must be in the state of supplying oxygen into the first gaschamber 1. On the contrary, when the oxygen concentration signal V₀₂ ishigher than the setting value V_(s), the oxygen pumping is in the stateof evacuating oxygen from the first gas chamber 1. The diagnosis inwhich these oxygen pumping directions, the difference (V₀₂−V_(s)) of theoxygen concentration signal in the gas chamber from the setting value,and the oxygen pumping current signal V_(ip) are compared, is carriedout to be judged as in order/disorder. These diagnosis operations can beput in practice using an analog circuit and more complicated diagnosisoperations will be possible by using a microcomputer.

Another sensor structure of the invention is shown in FIG. 3. In thissensor structure, because the oxygen pump cell (8 a, 8 b) shown in FIG.1 is eliminated, the oxygen pumping is given by the gas reforming pumpcell (9 a, 9 b). Furthermore, other different point from the sensorstructure of FIG. 1 is that the oxygen concentration signal in the firstgas chamber 1 is not fed back for the control of oxygen concentration.That is, the oxygen pumping amount introduced is constant by the gasreforming pump cell.

In the sensor structure of FIG. 3, the oxygen concentration in the firstgas chamber is influenced to some extent by the fluctuation of that inthe exhaust gas because of having no oxygen control means. For thisreason, in that sensor structure, it is not preferred to directly use apotential difference V_(NOx) between the NO_(x) sensing electrode 6 andthe reference electrode 7 as the NO_(x) sensing output. It is desirableto measure a potential difference V′_(NOx) between the NO_(x) sensingelectrode 6 and the first oxygen sensing electrode 4. Alternatively, itis preferred to use a method of subtracting the potential difference V₀₂between the first oxygen sensing electrode 4 and the reference electrode7 from the potential difference V_(NOx) between the NO_(x) sensingelectrode 6 and the reference electrode 7.

The disorder in the sensor shown in FIG. 3 is fundamentally similar tothat in the sensor shown in FIG. 1 or FIG. 2. However, in this sensorstructure, because the oxygen pump cell (8 a, 8 b) for controlling theoxygen concentration is omitted, the oxygen pumping current i_(p) cannotbe used as the signal for the self-diagnosis. Thus, the self-diagnosismethod of the sensor disorder in this structure is given as follows:δV₀₂ obtained as a difference between the oxygen concentration signalV₀₂ in the first gas chamber 1 and the oxygen concentration signal V*₀₂in the second gas chamber 2 (oxygen concentration signal in thedetection atmosphere) is compared with the setting value, whereby atleast serious disorder can be detected. A fundamental embodiment of thisself-diagnosis logic is shown in FIG. 5.

However, it can be easily predicted that in the self-diagnosis logic asshown in FIG. 5, a little degradation in the gas sealing property ishard to be detected. Thus, the self-diagnosis logic shown in FIG. 6 isproposed. In this system, the oxygen concentration signal V₀₂ in thefirst gas chamber 1 and the oxygen concentration signal V*₀₂ in thesecond gas chamber 2 (oxygen concentration signal in the detectionatmosphere) are compared with map data at any time, and when the resultis in a position out of a predetermined deviation amount, the sensordisorder is detected. That is, as shown in FIG. 6, V₀₂ and V*₀₂monitored at any time are subjected to a mapping treatment (MESDAT), andthe position data are compared (COMPDAT) with map data (NORDAT) in aorderly state already memorized to determine the sensor disorder. Forexample, the map data (NORDAT) are obtained for the pumping currentvalue ip of the NO_(x) converting cell using V₀₂ and V*₀₂ as variablesas shown in FIG. 7. In the case that map data for i*_(p1) deviate fromthe area wherein a predetermined amount of error is allowed, the sensordisorder is detected.

By using similar map data, the gas tightness of the air duct, in whichthe reference electrode 7 is disposed, can be diagnosed. For example, asshown in FIG. 8, δV₀₂ data obtained as a difference between the oxygenconcentration signal V₀₂ in the first gas chamber 1 and the oxygenconcentration signal V*₀₂ in the second gas chamber 2 (oxygenconcentration signal in the detection atmosphere) are obtained fori*_(p) with V*₀₂ as shown in FIG. 9. In the case that the map data fori*_(p1) deviate from the area that an error of a predetermined amount isallowed, the sensor disorder is detected.

It is obvious that the self-diagnosis method in the sensor structureshown in FIG. 3 can be applied to that of FIG. 1. Furthermore, in thesensor structure of the invention, in addition that the measurement ofthe concentration of detection gases and the self-diagnosis of thesensor can be carried out, the oxygen concentration in the detection gasatmosphere is directly detected and can be utilized for other systems.

The cross-sectional structures of the sensors of the preferredembodiments of the invention are shown in FIG. 1 to FIG. 3. As apreferred manufacturing method of these sensors, green sheet laminationof oxygen ion conductive solid electrolyte is used. As the oxygen ionconductive solid electrolyte, yttria (Y₂O₃)-added zirconia is usuallyused. The amount of yttria additive is generally from 3 to 8 mol %. Asthe additive substance, magnesia (MgO), ceria (CeO), etc., may be usedin place of yttria.

For the green sheet of zirconia, zirconia powder is first prepared as araw material. The zirconia powder added predetermined amount of Y₂O₃,MgO, etc. is used.

In the case of manufacturing the sensors of FIG. 1 to FIG. 3, pastes forvarious electrodes (4, 5, 6, 7, 8 a, 8 b, 9 a, 9 b, 10) are coated onthe zirconia green sheet by screen printing, etc. Lead conductors fortaking out signals from the various electrodes, insulating layers,heaters, etc. are also separately formed by printing. After screenprinting, the green sheets of the respective layers are laminated andpressed with heating. The laminated body is decreased at about 600° C.and sintered at 1,400° C. or higher. Finally, lead wires of Pt, etc.,are welded to lead terminals, and the resulting sensor is provided formeasurement.

The following materials are selected for the electrodes. For the oxygensensing electrodes 4, 5, the reference electrode 7, the oxygen pumpelectrodes 8 a, 8 b, and the gas reforming counter electrode 9 b, Ptactive only to oxygen is used. It is essential that the gas reformingelectrode 9 a and the gas sensing electrode 6 have an activity to atleast target gases and oxygen. In the case of the NO_(x) sensor, Rh,Pt—Rh alloy, etc. are used for the gas reforming electrode (NO_(x)converting electrode). For the NO_(x) sensing electrode, various metalmaterials and metal oxide materials, such as Rh, Pt—Rh alloy, NiCr₂O₄,MgCr₂O₄, Cr₂O₃, etc., are used.

In the sensors of the structures shown in FIG. 1 to FIG. 3, a heatingmeans is not shown, but in practically using the sensor, a heatersubstrate is usually added to those structures. From the viewpoints ofheat transfer and temperature control, the heater substrate is desiredto be bonded to the sensor substrate portion integratedly. Furthermore,from the viewpoints of the temperature distribution and relaxation ofthermal strain in the laminated sensor structure, it is preferred tobond heater substrates on both the surfaces (upper and lower surfaces inthe figure) of the sensor as shown in FIG. 10.

For the driving method of the sensors of the invention, a voltagevariable-type DC power supply 15 is used for the oxygen pump cell asdescribed above. The voltage of the power supply 15 is controlled byfeeding back the electromotive force between the first oxygen sensingelectrode 4 and the reference electrode 7.

On the other hand, the gas reforming pump cell is connected to a DCconstant voltage power supply for constantly applying a predeterminedvoltage. Considering gas reforming and oxygen pumping capability, theoptimum applied voltage is selected.

The various signals of the sensor of the invention except the pumpingcurrent are measured as the potential difference between the electrodes.The oxygen concentrations in the first gas chamber 1 and detectionatmosphere (the second gas chamber 2) are measured as concentrationelectromotive forces generated between the oxygen sensing electrodes 4,5 and the reference electrode 7 respectively. On the other hand, thepotential difference between the gas sensing electrode 6 and thereference electrode 7 or the first oxygen sensing electrode 4 is due toa mixed potential. It must be considered that the concentrationelectromotive force is a potential difference due to concentrationdifference of the same kind of gas. For these potential measurements, asa simple method, a potentiometer (circuit) is used, and in this case, itis preferred from an accuracy standpoint that the potentiometer has aninput impedance of at least three figures higher than the electrodeimpedance. On the other hand, the oxygen pumping current is measured byan ampere meter, but in the case of the present invention, the pumpingcurrent is taken out as the voltage signal.

The self-diagnosis method of the invention is explained in detail withreference to the following Examples.

EXAMPLE 1

A NO_(x) sensor having a structure shown in FIG. 10 was prepared by afollowing method. Pt electrodes (4, 5, 7, 8 a, 9 b, 9 b) and Pt—Rh alloyelectrodes (6, 9 a) were printed on green sheets of Y₂O₃-doped zirconia,followed by lamination and press into the above-described sensorstructure. The laminated body was sintered at a temperature of 1,400° C.to prepare a sensor element. The sensor was heated and maintained at600° C. by using heaters 17 a, 17 b, and NO_(x) sensing outputs fordetection gases having various oxygen and NO_(x) concentrations weremeasured.

As the NO_(x) output, a potential difference between the NO_(x) sensingelectrode 6 and the first oxygen sensing electrode 4 was measured. Theresults are shown in Table 1. In this case, the feed-back control of theoxygen concentration signal in the first gas chamber 1 to a variablepower supply of an oxygen pump cell was carried out. Since the oxygenconcentration in the first gas chamber was maintained at a predeterminedoxygen concentration (in this case, 2.0%) even if the oxygenconcentration in the detection gases was changed, it was confirmed thatthe influence of the interference gas (in this case, an oxygen gas) inthe detection gases was restrained, and the measurement of the NO_(x)concentration as carried out with high accuracy.

TABLE 1 Oxygen concentration NO_(x) concentration NO_(x) sensing indetection gases in detection gases output (%) (ppm) (mV) 0.1 50 60 1.050 57 1.5 50 57 4.0 50 61 9.0 50 62 15.0 50 58 20.9 50 59

On the other hand, a self-diagnosis circuit of the principle shown inFIG. 4 was prepared and connected to the sensor together with a drivingcircuit and a measurement circuit of the sensor. For the case of makingthe oxygen concentration in the detection gases 2% equal to the settingoxygen concentration in the first gas chamber 1, the order/disorderdiagnosis judgment was confirmed. That is, based on an electromotiveforce between the first oxygen sensing electrode 4 and the referenceelectrode 7, the oxygen pump cell-(8 a, 8 b) is driven so that theoxygen concentration in the first gas chamber 1 is always 2%.Accordingly, it becomes unnecessary to have oxygen pump cell work whenthe oxygen concentration in the first gas chamber 1 is 2%. At the sametime, it can be found that the oxygen concentration in the detectiongases is 2% from an electromotive force between the second oxygensensing electrode 5 and the reference electrode 7. In this case, theoxygen pumping current becomes zero. It was confirmed from the signaloutput of the diagnosis circuit, therefore, that the first oxygensensing electrode 4 in the first gas chamber 1 and the oxygen pumpingcell are worked in order.

EXAMPLE 2

A NO sensor similar to that in Example 1 was prepared and connected withthe above-described self-diagnosis circuit. The oxygen concentration inthe detection gases was set to 0.1%. In this case, it was confirmed fromthe output signal of the diagnosis circuit that the output current ofthe oxygen pump flew at about 400 μA in the positive direction so as tosupply the oxygen to the gas chamber 1. However, the potential of thefirst oxygen sensing electrode in the first gas chamber 1 did notincrease to the potential corresponding to 2% of the redetermined oxygenconcentration, that is, the disorder of the sensor was shown. As theresult of detailed inspection, cracking was found at a part of thezirconia substrate of the sensor element.

What is claimed is:
 1. A combined gas sensor comprising: a first gaschamber formed by first and second oxygen ion conductive solidelectrolytes; a first gas inlet having a diffusion resistance forrestricting a flow amount of gases entering the first gas chamber; afirst oxygen sensing electrode active only to oxygen, which is disposedon a surface of the first oxygen ion conductive solid electrolytesubstrate inside the first gas chamber; a gas sensing electrode activeto at least target gases and oxygen, which is disposed on the samesurface of the first electrolyte substrate as the first oxygen sensingelectrode inside the first gas chamber; a reference electrode active tooxygen, which is disposed on the first solid electrolyte substrate suchthat it is brought into contact with an atmospheric air separated fromthe first gas chamber; an oxygen pump cell for controlling oxygenconcentration in the first gas chamber, of which electrodes are made ofmaterials active only to oxygen and disposed on the second solidelectrolyte substrate facing to the first solid electrolyte substrate; agas reforming pump cell for electrochemically converting or decomposingthe gases, of which a gas reforming electrode is active to oxygen andthe target gases, and disposed on the second solid electrolyte substrateinside the first gas chamber so as to face the gas sensing electrode,and a gas reforming counter electrode disposed on the second solidelectrolyte substrate outside the gas chamber such that it is broughtinto contact with the atmospheric air; and a second oxygen sensingelectrode active only to oxygen, which is disposed on the first solidelectrolyte substrate such that it is directly exposed to a detectiongas atmosphere.
 2. The combined gas sensor according to claim 1, whereinthe second oxygen sensing electrode active only to oxygen, which isdisposed on the first solid electrolyte substrate such that it isdirectly exposed to the detection gas atmosphere, is disposed in asecond gas chamber having a gas inlet and being adjacent to an air ductin which the reference electrode is disposed, and the oxygenconcentration in the detection gas atmosphere is measured usingconcentration electromotive force between said second oxygen sensingelectrode and said reference electrode.
 3. The combined gas sensoraccording to claim 1, wherein the target gases are nitrogen oxides madeof NO and NO₂ as main constituents.
 4. A combined gas sensor comprising:a first gas chamber formed by first and second oxygen ion conductivesolid electrolytes; a first gas inlet having a diffusion resistance forrestricting a flow amount of gases entering the first gas chamber; afirst oxygen sensing electrode active only to oxygen, which is disposedon a surface of the first oxygen ion conductive solid electrolytesubstrate inside the first gas chamber; a gas sensing electrode activeto at least target gases and oxygen, which is disposed on the samesurface of the first electrolyte substrate as the first oxygen sensingelectrode inside the first gas chamber; a reference electrode active tooxygen, which is disposed on the first solid electrolyte substrate suchthat it is brought into contact with an atmospheric air separated fromthe first gas chamber; a gas reforming pump cell for electrochemicallyconverting or decomposing the gases, of which a gas reforming electrodeis active to oxygen and the target gases and disposed on the secondsolid electrolyte substrate inside the first gas chamber so as to facethe gas sensing electrode, and a gas reforming counter electrodedisposed on the second solid electrolyte substrate outside the first gaschamber such that it is brought into contact with the atmospheric air;and a second oxygen sensing electrode active only to oxygen, which isdisposed on the first solid electrolyte substrate such that it isdirectly exposed to a detection gas atmosphere.
 5. The combined gassensor according to claim 4, wherein the second oxygen sensing electrodeactive only to oxygen, which is disposed on the first solid electrolytesubstrate such that it is directly exposed to the detection gasatmosphere, is disposed in a second gas chamber having a gas inlet andbeing adjacent to an air duct in which the reference electrode isdisposed, and the oxygen concentration in the detection gas atmosphereis measured using concentration electromotive force between said secondoxygen sensing electrode and said reference electrode.
 6. The combinedgas sensor according to claim 4 wherein the target gases are nitrogenoxides made of NO and NO₂ as main constituents.
 7. A combined gas sensorcomprising: an oxygen pump cell for controlling oxygen concentration ina first gas chamber, of which electrodes are made of materials activeonly to oxygen and disposed on a second solid electrolyte substratefacing a first solid electrolyte substrate; an oxygen sensing cellcomprising a first oxygen sensing electrode active only to oxygen, whichis disposed on a surface of the first solid electrolyte substrate in thegas chamber and a reference electrode which is disposed on the firstsolid electrolyte substrate such that it is brought into contact with anatmospheric air separated from the first gas chamber; and a gasreforming pump cell for electrochemically converting or decomposingtarget gases, of which a gas reforming electrode active to oxygen andthe target gases is disposed on the second solid electrolyte substratein the gas chamber so as to face a gas sensing electrode, and a gasreforming counter electrode is disposed on the second solid electrolytesubstrate outside the gas chamber such that it is brought into contactwith an atmospheric air, and a second oxygen sensing electrode activeonly to oxygen, which is disposed on the first solid electrolytesubstrate such that it is directly exposed to a detection gasatmosphere; and characterized by having a cell driving circuit wherein aDC variable power supply for controlling the oxygen concentration in thegas chamber at a constant level by feed-back control of pumping currentsto an applied voltage of the oxygen pump cell according to anelectromotive force between the first oxygen sensing electrode and thereference electrode being as a signal output is connected to the oxygenpump cell, and a DC constant voltage power supply for making an oxygenpumping current of the gas reforming pump cell constant and making anelectrochemical decomposition or conversion amount of the target gasconstant is connected to the gas reforming pump cell.
 8. The combinedgas sensor according to claim 7, wherein the oxygen concentration in thedetection gas atmosphere is directly measured by the electromotive forcebetween the second oxygen sensing electrode and the reference electrode,such that self-diagnosis of sensor disorder can be carried out using asignal output of said oxygen concentration.
 9. The combined gas sensoraccording to claim 7, wherein a signal output due to the oxygenconcentration in the detection gas atmosphere, the signal output due tothe oxygen concentration in the first gas chamber, a pumping current ofthe oxygen pump cell, and a pumping current of the gas reforming cellcan be detected as voltage signals, and self diagnosis of disorder ofthe oxygen sensing cell with the first oxygen sensing electrode and thereference electrode, the oxygen pump cell, and the gas reforming pumpcell, or the gas tightness of the gas chamber and the air duct, can becarried out by comparative operations among said voltage signals. 10.The combined gas sensor according to claim 7, wherein the gas sensor hasmeans for self-diagnosis of disorder of the oxygen sensing cell with thefirst oxygen sensing electrode and the reference electrode, the oxygenpump cell, and the gas reforming pump cell, or about gas tightness ofthe gas chamber and the air duct, and said self-diagnosis can be carriedout from a comparison and judgment of a signal output due to the oxygenconcentration in the detection atmosphere, detection data using saidsignal output, and previously memorized map data.
 11. A combined gassensor comprising: an oxygen pump cell for controlling oxygenconcentration in a gas chamber, of which electrodes are made ofmaterials active only to oxygen and disposed on a second solidelectrolyte substrate facing a first solid electrolyte substrate; afirst oxygen sensing cell comprising a first oxygen sensing electrodeactive only to oxygen, which is disposed on a surface of the first solidelectrolyte substrate in the gas chamber and a reference electrode whichis disposed on the first solid electrolyte substrate such that it isbrought into contact with an atmospheric air separated from the gaschamber; a gas reforming means for electrochemically converting ordecomposing target gases; a second oxygen sensing cell comprising asecond oxygen sensing electrode active only to oxygen, which is disposedon the first solid electrolyte substrate such that it is directlyexposed to a detection gas atmosphere and said reference electrode; agas sensing cell comprising a gas sensing electrode active to at leasttarget gases and oxygen, which is disposed on the same surface of thefirst electrolyte substrate as the first oxygen sensing electrode insidethe gas chamber; a cell driving circuit connected to the oxygen pumpcell, wherein a DC variable power supply for controlling the oxygenconcentration in the gas chamber at a constant level by feed-backcontrol of pumping currents to an applied voltage of the oxygen pumpcell according to an electromotive force between the first oxygensensing electrode and the reference electrode being as a signal output;and a self-diagnosis means for comparing a current signal of said oxygenpump cell, a first oxygen concentration signal in said gas chamberdetected by said first oxygen sensing cell and/or a second oxygenconcentration signal in said detection gas atmosphere detected by saidsecond oxygen sensing cell with a map data predetermined from a firstoxygen concentration signal in said gas chamber detected by said firstoxygen sensing cell and a second oxygen concentration signal in saiddetection gas atmosphere detected by said second oxygen sensing cell tojudge a disorder of the sensor.
 12. The combined gas sensor according toclaim 11, wherein said self-diagnosis means comprises: an input andvoltage comparing portion connected with said current signal of saidoxygen pump cell, said first oxygen concentration signal in said gaschamber detected by said first oxygen sensing cell and said secondoxygen concentration signal in said detection gas atmosphere detected bysaid second oxygen sensing cell; a controlling portion for a diagnosismode; a signal operation portion, in which said map data is memorized;and an output portion of diagnosis results.
 13. The combined gas sensoraccording to claim 11, wherein: said gas reforming means comprises a gasreforming electrode active to oxygen and the target gases disposed onthe second solid electrolyte substrate in the gas chamber so as to facea gas sensing electrode, and a gas reforming counter electrode disposedon the second solid electrolyte substrate outside the gas chamber suchthat it is brought into contact with an atmospheric air.
 14. A combinedgas sensor comprising: a gas reforming pump cell for electrochemicallyconverting or decomposing target gases, of which a gas reformingelectrode active to oxygen and the target gases is disposed on a secondsolid electrolyte substrate in a gas chamber so as to face a gas sensingelectrode, and a gas reforming counter electrode is disposed on thesecond solid electrolyte substrate outside the gas chamber such that itis brought into contact with an atmospheric air; a first oxygen sensingcell comprising a first oxygen sensing electrode active only to oxygen,which is disposed on a surface of a first solid electrolyte substrate inthe gas chamber and a reference electrode which is disposed on the firstsolid electrolyte substrate such that it is brought into contact with anatmospheric air-separated from the gas chamber; a second oxygen sensingcell comprising a second oxygen sensing electrode active only to oxygen,which is disposed on the first solid electrolyte substrate such that itis directly exposed to a detection gas atmosphere and said referenceelectrode; a gas sensing cell comprising a gas sensing electrode activeto at least target gases and oxygen, which is disposed on the samesurface of the first electrolyte substrate as the first oxygen sensingelectrode inside the gas chamber; and a self-diagnosis means forcomparing a pumping current value of said gas reforming pump cell, afirst oxygen concentration signal in said gas chamber detected by saidfirst oxygen sensing cell and/or a second oxygen concentration signal insaid detection gas atmosphere detected by said second oxygen sensingcell, with a map data predetermined from a first oxygen concentrationsignal in said gas chamber detected by said first oxygen sensing celland a second oxygen concentration signal in said detection gasatmosphere detected by said second oxygen sensing cell to judge adisorder of the sensor.
 15. The combined gas sensor according to claim14, wherein said self-diagnosis means comprises: an input and voltagecomparing portion connected with said current signal of said gasreforming pump cell, said first oxygen concentration signal in said gaschamber detected by said first oxygen sensing cell and said secondoxygen concentration signal in said detection gas atmosphere detected bysaid second oxygen sensing cell; a controlling portion for a diagnosismode; a signal operation portion, in which said map data is memorized;and an output portion of diagnosis results.